libibmad: add per-port SA key configuration

The libibmad SA query helpers have no way to supply a nonzero SM_Key.
Calls through sa_rpc_call(), including ib_path_query_via() and
ib_node_query_via(), therefore always issue untrusted requests with
SM_Key 0.

This prevents those helpers from querying records that require a
trusted request, such as PKeyTableRecord, and from working with subnet
managers configured to reject untrusted SA requests. Callers currently
have to bypass the helpers and implement the UMAD transaction
themselves.

Add per-port sa_key_set() and sa_key_get() functions following the
existing SMP M_Key API. Store the key in the opaque ibmad_port and copy
it into rpc.mkey when constructing an SA request. Newly opened ports
remain initialized to key 0, preserving existing behavior unless a
caller explicitly sets a key.

Export the new functions under IBMAD_1.6, update the libibmad ABI and
distribution packaging, and document the API.

Signed-off-by: Jonathan Süssemilch Poulain <jpoulain@coreweave.com>